User-Defined Types in C++

Surprising no one, user-defined types are types that the user can define.

Enumerations

The simplest.
The values that an enumeration can take are restricted to a set of possible values.
Excellent for modelling categorical concepts.

Classes

More fully featured types that give you flexibility to pair data and functions.
Classes that only contain data are called “plain-old-data” classes.

Unions

Boutique user-defined type.
All members share the same memory location.
Unions are dangerous and easy to misuse.

Enumeration Types

Declare enums using keyword enum class, followed by the type name and a listing og the values it can take.
The values are arbitrary alphanumeric strings that will represent whatever categories you want to represent.
Under the hood these values are just integers.
However, these allow you to write safer, more expressive code by using programmer-defined types rather than integeres that could mean anything.
To intialize an enumeration variable to a value, use the name of the type, followed by two colons (::) and the desired value.

Plain-old-data Classes (PODs)

Classes are user-defined types that contain data and functions.
They are the heart and soul of C++ that allow you to express yourself.
Plain-old-date classes are simple containers.
Think of them as a sort of array of elements of potentially DIFFERENT types.
Each element within a class is called a MEMBER.
PODs are C compatible, they are efficiently represented in memory, and you can employ machine instructions that are highly efficient to copy/move them.
C++ also guarantees that members will be sequential in memory.
In general, you should order members from largest to smallest within POD definitions.
Every POD begins with the keyword struct followed by the PODs desired name.
Next, list the members types and names.
So in usertypes.cpp, you can see we created a POD by using struct Book{}.
To access or set one of those members, you use dot-operators.
So if I create a Book object Dune, I can set the page count by using dune.pages

Working examples

These are rendered from the source stored beside this drawer's notes.

user-defined-types.cpp

raw source ↗

#include <cstdio>

// The purpose of the below code is to demonstrate the creation of an enum class, example altered from C++ crash course.
enum class Race {
    Human,
    Turian,
    Salarian,
    Krogan,
    Asari,
    Geth,
    Quarian,
};

// The purpose of the below code is to demonstrate initializing an enumeration variable to a value.
Race garrus_race = Race::Turian;

//The below function's purpose is to demonstrate the use of a switch statement with an enumeration class.
void ME_party_member_quote(){
    Race character_race = Race::Human;

    switch(character_race) {
        case Race::Human: {
            printf("I'm Commander Shepherd, and this is my favorite store on the Citadel.\n");
        } break;
        case Race::Turian: {
            printf("I'm Garrus Vakarian. It's so much easier to see the world in black and white. Gray? What do I do with that?\n");
        } break;
        case Race::Salarian: {
            printf("I'm Mordin. I am the very model of a scientist salarian, I’ve studied species turian, asari, and batarian.\n");
        } break;
        case Race::Krogan: {
            printf("I'm Urdnot Wrex. Why shoot something once when you can shoot it 46 more times?\n");
        } break;
        case Race::Asari: {
            printf("I'm Liara T'Soni. It doesn't have any weapons, Shephard. It's a taxi, it has a fare meter!\n");
        } break;
        case Race::Geth: {
            printf("We are Legion. Geth do not infilitrate. Geth do not..intentionally infiltrate.\n");
        } break;
        case Race::Quarian: {
            printf("I am Tali'Zorah vas Normandy. I am getting drunk very carefully. Turian brandy, triple filtered, then introduced into the suit through an emergency induction port.\n");
        } break;
        default: {
            printf("Error: unknown race!\n");
        }
    }   
}

// The purpose of the below code is to demonstrate the creation of a plain-old-data class using struct and then accessing it with a function.

struct Book {
    char name[256];
    int year;
    int pages;
    bool hardcover;
};

void favorite_book(){
    Book dune;
    dune.pages = 704;
    printf("Dune has %d pages in it.\n", dune.pages);
}


int main(){
    ME_party_member_quote();
    favorite_book();
}